In AVNRT, a premature beat typically causes a unidirectional block in the fast pathway; the impulse travels down the slow pathway.

• A. Bidirectional block in the slow pathway
• B. Unidirectional block in the fast pathway; conduction down the slow pathway
• C. Bidirectional block in the fast pathway
• D. No block in either pathway

Answer: (B)

AVNRT relies on two pathways within the AV node with different conduction properties. A premature beat often encounters the fast pathway still refractory for antegrade conduction while the slow pathway has recovered and can conduct. This creates a unidirectional block in the fast pathway, forcing the impulse to travel down the slow pathway to the ventricles. By the time it reaches the lower pathway, the fast pathway has recovered enough to conduct retrogradely, sending the impulse back up to the atria and completing a reentrant loop that sustains the tachycardia. If the slow pathway couldn’t conduct antegradely, or if the fast pathway were blocked in both directions, the reentrant circuit wouldn’t form. If there were no block at all, the rhythm would remain normal rather than producing AVNRT.